The rosy gene on chromosome III of Drosophila melanogaster is responsible for the biosynthesis of the enzyme, xanthine dehydrogenase (XDH). A large body of genetic data has been accumulated for this region, making it one of the most thoroughly characterized loci of any differentiated organism, from the genetic standpoint. Studies are being carried out to provide biochemical information on XDH to complement the genetic studies, and to extend our knowledge of the rosy region to the molecular level. Experiments are designed to: 1. Determine the locaton of XDH structural variants on the polypeptide chain by peptide mapping. 2. Provide information of the physical and chemical properties of normal and defective enzymes. 3. Examine available rosy mutants for the presence of possible chain termination and deletion mutations with the XDH structural gene.